1. Field of the Invention
The present invention relates to static memory devices, in particular in-circuit programmable static memory devices.
2. State of the Art
There are a significant number of integrated circuit applications that require some sort of electrically programmable memory within the integrated circuit chip. These applications include, for example, applications which require several bits of programmable memory (e.g., programming identification numbers), to applications which require several megabits of programmable memory (e.g., storing operating code).
In the prior art, a wide variety of technologies have been used for implementing programmable memory within integrated circuits. For example, these include floating-gate non-volatile memories and anti-fuses.
One problem with most prior art approaches to providing programmable memory is that extra wafer processing is required to implement them. This increases the production cost. The extra wafer processing is particularly difficult to justify when only relatively small amounts of electrically programmable memory are required on each integrated circuit. It is very advantageous to identify a programmable element that could be produced within the baseline logic integrated circuit process, thus eliminating any additional wafer processing cost.
There have been some attempts to develop programmable elements that can be produced within the baseline logic integrated circuit process. One such "zero-cost" approach that has been used in the past is to create a fuse out of the existing polysilicon or metal layers, and then "blow" the fuse by passing a large programming current. The dissipated heat causes local melting and vaporization of the fuse material, causing the fuse to transition from a relatively low resistance to an open circuit.
There are several significant problems that limit the applicability of the prior art polysilicon or metal fuses. The most fundamental problem is the damage that takes place when the fuse is blown. The blowing of the fuse is usually associated with vaporization of the fuse material, leading to catastrophic rupture of any layers of dielectric or metal that would normally be on top of the fuse in a conventional integrated circuit process. The rupture of the overlying layers represents a significant reliability hazard, as it may cause circuit malfunction directly, or by allowing external contaminants to gain ingress to the integrated circuit. The most common approach to this problem is to create a "passivation opening" over the top of the fuse to ensure that there are no overlying layers present. In this way any vaporized metal can escape readily without creating damage. The problem with this approach is that the pad opening destroys the integrity of the overlying "passivation" layer on the die, and so will allow external contaminants to enter the die and cause long-term reliability problems. In practice, when "passivation openings" are used, it is necessary to package the finished product in an expensive "hermetic" package. The package itself will protect the die from external contamination.
Other secondary problems are frequently experienced with polysilicon or metal fuses. The sheet resistance of a doped polysilicon layer is typically in the range 25-60 ohms/square. The power dissipated in the fuse is given by V.sup.2 /R, where V is the voltage applied to the fuse, and R is the fuse resistance. For typical fuse designs, the voltage V required to generate sufficient heat to destroy the fuse will be higher than the power supplies (2.5-3.3V) used by advanced integrated circuits. This necessitates that an extra programming power supply must be provided, and in some cases, special high voltage transistors must be included in the process to handle this voltage. Such an addition to the process undercuts the whole aim of adding programmability at no extra wafer processing cost.
Metal fuses have the opposite problem. The sheet resistance of the metal is very low (typically 40-80 milliohms/square), and so the whole fuse will have a resistance of less than an ohm. The voltage required for programming will therefore be very low. However, the power dissipation can be given as I.sup.2 *R, where I is the current passing through the fuse. Due to the low fuse resistance, a very large programming current will be required to dissipate sufficient power to program the fuse. The programming current must be steered to the required fuse by a series of select transistors. In order to accommodate the very high programming current, these select transistors will need to be very large, hence occupying a significant amount of die area, and increasing product cost.